# Cardiac Amyloidosis Scintigraphy

## Background on Cardiac Amyloidosis

### Types of Cardiac Amyloidosis

Cardiac amyloidosis results from the deposition of misfolded protein fibrils in the myocardium, and the two major types have fundamentally different biology, prognosis, and treatment. Transthyretin amyloidosis (ATTR) involves misfolded transthyretin protein depositing in the heart and exists in two forms. Wild-type ATTR (ATTRwt), formerly called senile systemic amyloidosis, is an age-related condition that predominantly affects men over 75 years. Hereditary ATTR (ATTRv) results from autosomal dominant mutations in the TTR gene with variable age of onset; the Val122Ile mutation is particularly notable because it is carried by approximately 3 to 4% of African Americans. Light-chain amyloidosis (AL) is caused by misfolded immunoglobulin light chains (lambda or kappa) and is associated with an underlying plasma cell dyscrasia. AL amyloidosis follows a more aggressive clinical course, requires hematologic treatment such as chemotherapy or stem cell transplant, and carries a prognosis driven primarily by the degree of cardiac involvement. Distinguishing ATTR from AL is critical because the prognosis, treatment, and diagnostic approach differ substantially, and bone scintigraphy is highly specific for ATTR.

### Clinical Presentation

Cardiac amyloidosis commonly presents as heart failure with preserved ejection fraction (HFpEF), particularly in elderly patients. A restrictive cardiomyopathy pattern is typical, and left ventricular wall thickening is often misdiagnosed as hypertrophic cardiomyopathy. A hallmark clue is voltage-mass mismatch, in which the ECG shows low voltage despite thick ventricular walls. Atrial fibrillation and conduction disease are frequently present. Carpal tunnel syndrome may precede cardiac symptoms by years in ATTR, and spinal stenosis is an associated finding in ATTRwt.

## Bone-Avid Radiotracer Scintigraphy

### Radiopharmaceuticals

Three bone-seeking Tc-99m-labeled agents are used for cardiac amyloidosis scintigraphy. Tc-99m pyrophosphate (PYP) is the most commonly used tracer in North America. Tc-99m DPD (3,3-diphosphono-1,2-propanodicarboxylic acid) is used in Europe, and Tc-99m HMDP (hydroxymethylene diphosphonate) is used in Europe and some other regions. All three are bone-seeking phosphonate agents that also bind to ATTR amyloid deposits in the heart. The mechanism of uptake involves binding to the calcium content within ATTR amyloid fibrils, specifically the microcalcifications present within these fibrils.

### Why These Tracers Work for ATTR

ATTR amyloid fibrils contain microcalcifications that bind bone-seeking phosphonate tracers, producing significant myocardial uptake. AL amyloid fibrils have substantially less calcium content and therefore demonstrate less avid uptake, usually grade 0 or 1. This calcium hypothesis explains why bone tracers are highly specific for ATTR over AL amyloidosis and forms the basis of the diagnostic approach. However, some cases of AL amyloidosis can show mild (grade 1) uptake, which is why AL must be excluded before a positive bone scan can be used to confirm ATTR.

## Imaging Protocol

### Tc-99m PYP Protocol (ASNC Recommendations)

The standard protocol involves intravenous injection of 10 to 25 mCi (370 to 925 MBq) of Tc-99m PYP with a minimum 1-hour wait post-injection, though some protocols use a 3-hour delay to reduce blood pool interference. One-hour imaging is more convenient but requires blood pool subtraction or SPECT to confirm that uptake is truly myocardial. Three-hour imaging allows greater blood pool clearance, making planar semi-quantitative analysis more reliable. Planar imaging consists of an anterior view of the chest with region-of-interest (ROI) analysis. SPECT or SPECT/CT is recommended at 1 hour and is essential to confirm that apparent myocardial uptake is not simply blood pool activity.

### Image Interpretation

#### Visual Grading (Perugini Scale)

The Perugini visual grading scale classifies uptake into four grades. Grade 0 shows no myocardial uptake with rib uptake present and is negative for ATTR. Grade 1 shows myocardial uptake less than rib uptake and is considered equivocal, as it may represent either ATTR or AL. Grade 2 shows myocardial uptake equal to rib uptake and is positive for ATTR when AL has been excluded. Grade 3 shows myocardial uptake greater than rib uptake with ribs barely visible, representing strongly positive findings for ATTR.

| Grade | Myocardial vs Rib Uptake | Interpretation | Next Step |
|---|---|---|---|
| 0 | No myocardial uptake | Negative for ATTR | Consider other diagnoses (does not exclude AL) |
| 1 | Myocardial < Rib | Equivocal | Consider biopsy; may be early ATTR or AL |
| 2 | Myocardial = Rib | Positive for ATTR (if AL excluded) | TTR gene sequencing |
| 3 | Myocardial > Rib (ribs faint) | Strongly positive for ATTR (if AL excluded) | TTR gene sequencing |

#### Semi-Quantitative Analysis: Heart-to-Contralateral (H/CL) Ratio

The H/CL ratio is calculated by drawing an ROI over the heart and a mirror ROI over the contralateral chest. A ratio of 1.5 or greater at 1 hour, or 1.3 or greater at 3 hours, is considered positive for cardiac ATTR amyloid uptake. This measurement provides an objective and reproducible quantification and should be used in conjunction with visual grading and SPECT confirmation.

#### SPECT Imaging

SPECT imaging is essential at 1-hour protocols to confirm that uptake is truly myocardial and not blood pool activity. Blood pool activity arises because stannous pyrophosphate causes in vivo RBC labeling, and this blood pool activity can mimic myocardial uptake on planar images. SPECT resolves this by showing whether activity localizes to the myocardial wall versus the ventricular blood pool cavity. At 3 hours, blood pool has largely cleared, making SPECT less critical but still recommended. SPECT/CT additionally provides attenuation correction and anatomic correlation.

## Diagnostic Algorithm

### Step 1: Exclude AL Amyloidosis

Before diagnosing ATTR based on scintigraphy, AL amyloidosis must be excluded. This requires obtaining serum free light chains (kappa and lambda with ratio), serum protein electrophoresis (SPEP), and urine protein electrophoresis (UPEP) with immunofixation. If any of these studies are abnormal, AL amyloidosis cannot be excluded, and tissue biopsy is required, either endomyocardial biopsy or fat pad biopsy with Congo red staining and mass spectrometry. If all studies are normal, AL is effectively excluded with greater than 99% negative predictive value.

### Step 2: Interpret Bone Scintigraphy

Grade 2 or 3 uptake combined with normal serum and urine studies establishes a diagnosis of ATTR cardiac amyloidosis without the need for biopsy. This non-invasive diagnostic approach has greater than 99% specificity and positive predictive value for ATTR, as reported in the landmark study by Gillmore et al. in 2016. Grade 0 uptake excludes ATTR but does not exclude AL amyloidosis. Grade 1 uptake is equivocal and could represent early ATTR or AL; biopsy should be considered. Grade 1 uptake with abnormal light chain studies makes biopsy essential.

### Step 3: Subtyping ATTR

Once ATTR is diagnosed, it is critical to determine whether it is wild-type or hereditary. TTR gene sequencing identifies mutations such as Val122Ile, Thr60Ala, and Val30Met. If no mutation is found, the diagnosis is ATTRwt, managed with supportive care and tafamidis. If a mutation is present, the diagnosis is ATTRv, which requires genetic counseling, family screening, and targeted therapies including tafamidis.

## Treatment Implications

### Tafamidis (Vyndaqel/Vyndamax)

Tafamidis is a TTR stabilizer that prevents tetramer dissociation and subsequent amyloid fibril formation. It received FDA approval for ATTR cardiomyopathy (ATTR-CM) based on the ATTR-ACT trial, which demonstrated reductions in both mortality and hospitalization in patients with NYHA class I through III ATTR-CM. Importantly, a scintigraphy-confirmed ATTR diagnosis qualifies patients for treatment without biopsy.

### Other Therapies

Additional therapeutic options include TTR gene silencers such as patisiran, inotersen, and vutrisiran, which are siRNA or antisense oligonucleotide agents that reduce TTR production. These were primarily studied in polyneuropathy, but cardiac benefits are emerging. Diflunisal serves as an off-label TTR stabilizer, and CRISPR-based gene editing approaches are under investigation.

### AL Amyloidosis Treatment

Treatment for AL amyloidosis targets the underlying plasma cell clone and includes chemotherapy, stem cell transplant, and daratumumab. Bone scintigraphy is not used for diagnosis of AL cardiac amyloidosis because of its low sensitivity for this subtype. Cardiac MRI and endomyocardial biopsy are the primary diagnostic tools for AL.

## Pitfalls and Limitations

### False Positives

Blood pool activity from in vivo RBC labeling by stannous pyrophosphate can mimic myocardial uptake on planar images, a problem that SPECT resolves. Recent myocardial infarction shows Tc-99m PYP uptake in necrotic myocardium (this agent was historically used for MI imaging), so imaging should be deferred at least 2 weeks post-MI. Left ventricular thrombus may accumulate tracer. Hydroxychloroquine cardiotoxicity can also cause PYP uptake.

### False Negatives

Early-stage ATTR may have grade 0 to 1 uptake before sufficient amyloid has deposited. AL amyloidosis is not reliably detected by bone scintigraphy, so a negative scan does not exclude AL cardiac amyloidosis. Aortic stenosis frequently coexists with ATTR in elderly patients, and both conditions should be evaluated.

### Technical Considerations

Adequate wait time of at least 1 hour (with 3 hours preferred for planar-only protocols) is essential. SPECT should always be obtained at 1-hour imaging to differentiate myocardial from blood pool uptake. The H/CL ratio can be affected by ROI placement, so the technique should be standardized.

<image>A diagnostic algorithm flowchart for non-invasive diagnosis of cardiac ATTR amyloidosis. Start with clinical suspicion (HFpEF, unexplained LV thickening, low-voltage ECG). Step 1: obtain serum free light chains, SPEP/UPEP with immunofixation. Branch based on results: if abnormal, proceed to tissue biopsy. If normal (AL excluded), proceed to Tc-99m PYP/DPD/HMDP scintigraphy. Grade 2-3 uptake with normal labs = ATTR diagnosed non-invasively. Grade 0 = ATTR excluded. Grade 1 = equivocal, consider biopsy. After ATTR diagnosis: TTR gene sequencing to distinguish ATTRwt from ATTRv.</image>

<image>A panel of Tc-99m PYP scintigraphy images demonstrating the four visual grades. Grade 0: no myocardial uptake with normal rib uptake. Grade 1: faint myocardial uptake less than ribs. Grade 2: myocardial uptake equal to ribs. Grade 3: intense myocardial uptake with ribs barely visible. Include both planar anterior chest images and corresponding SPECT short-axis slices for each grade, showing how SPECT confirms myocardial wall uptake (grade 2-3) versus blood pool activity (which might mimic grade 1-2 on planar alone).</image>

<image>An illustration comparing the pathophysiology of ATTR versus AL cardiac amyloidosis and their different appearances on bone scintigraphy. Show ATTR amyloid fibrils with microcalcifications binding Tc-99m PYP (grade 2-3 uptake) versus AL amyloid fibrils with minimal calcium content showing no significant PYP uptake (grade 0-1). Emphasize that a positive bone scan requires exclusion of AL before confirming ATTR, and that a negative bone scan does NOT exclude AL amyloidosis.</image>

## Clinical Pearls

Tc-99m PYP scintigraphy can diagnose ATTR cardiac amyloidosis non-invasively with greater than 99% specificity, but only after AL amyloidosis has been excluded with serum free light chains, SPEP, and UPEP.

Grade 2 or 3 uptake combined with normal light chain studies establishes ATTR cardiac amyloidosis without biopsy. This practice-changing diagnostic approach was validated in the landmark Gillmore et al. 2016 study.

SPECT imaging is essential at 1-hour protocols because blood pool activity from in vivo RBC labeling (stannous pyrophosphate labels RBCs) can mimic myocardial uptake on planar images and cause false-positive interpretations.

A negative Tc-99m PYP scan does not exclude AL cardiac amyloidosis. Bone scintigraphy is insensitive for AL, and if clinical suspicion remains high, cardiac MRI and endomyocardial biopsy should be pursued.

The Val122Ile TTR mutation is present in 3 to 4% of African Americans. ATTR should be considered in Black patients presenting with HFpEF and left ventricular hypertrophy.

Diagnosis of ATTR enables treatment with tafamidis, which reduces mortality in ATTR cardiomyopathy, making accurate non-invasive diagnosis clinically impactful.

Always check for recent myocardial infarction before interpreting PYP scintigraphy, as acute myocardial necrosis causes PYP uptake that can confound interpretation.

## References

- Gillmore JD, et al. Nonbiopsy diagnosis of cardiac transthyretin amyloidosis. *Circulation*. 2016;133(24):2404-2412.
- Dorbala S, et al. ASNC/AHA/ASE/EANM/HFSA/ISA/SCMR/SNMMI expert consensus recommendations for multimodality imaging in cardiac amyloidosis. *J Nucl Med*. 2021;62(7):7S-26S.
- Maurer MS, et al. Tafamidis treatment for patients with transthyretin amyloid cardiomyopathy (ATTR-ACT). *N Engl J Med*. 2018;379(11):1007-1016.
- Bokhari S, et al. Tc-99m pyrophosphate scintigraphy for differentiating light-chain cardiac amyloidosis from transthyretin cardiac amyloidosis. *Circ Cardiovasc Imaging*. 2013;6(2):195-201.